This invention relates to a method and system for avoiding congestion in a network of switches (nodes) due to a mass calling event (MCE).
A terminating number (TN) is a destination number serviced by a terminating switch (which switch may be inside or outside a given switch network). A focussed MCE occurs where the rate of calls to any given TN (the "congestion TN") increases dramatically resulting in a large number of incomplete calls directed towards that number. For example, where a TN is a number for a ticket outlet, a focussed MCE may occur as soon as tickets for a popular concert go on sale.
Most circuit switched communications networks allocate a call path in a forward fashion in that they will begin routing of each call until the call is blocked by a network element which is "busy" (i.e., fully occupied). In such a network, a focussed MCE may quickly congest the entire network precluding calls to other TNs from being connected. Even if the focussed MCE does not congest the entire network, it will result in a waste of switching resources (causing unnecessary processing at each switch) and transmission resources (i.e., trunks between switches).
U.S. Pat. No. 5,295,183 issued March 15, 1994 to Langlois describes a system for coping with non-focussed MCEs (i.e., MCEs which are not caused by calls to one or a few specific TNs but result from general network congestion). In Langlois, switch occupancy rates are sent by each switch to a network processor (NP) on a periodic basis. When the occupancy rate of a switch exceeds a threshold, the NP sends back a recommendation to all switches in the network with an identifier of the congested switch and a call gap for calls to the congested switch. Upon receiving the recommendation, the switches begin timing a "call gapping time". When a switch receives a call that is directed toward the congested switch, if the call gapping time has elapsed, the call is routed as usual and the gapping time is reset. On the other hand, if a switch receives a call directed to the congested switch and the gap time has not elapsed, the call is blocked. When the NP receives new occupancy rates from the switches, it may broadcast a new recommendation. When the occupancy rate for a congested switch falls below a threshold, the NP may broadcast a message such that call gapping of calls directed to the previously congested switch ceases.
This invention seeks to cope with an MCE in a circuit switched communications network which allocates a call path in a forward fashion.